Smart way to generate permutation and combination of String

You should check this answer: Getting every possible permutation of a string or combination including repeated characters in Java

To get this code:

public static String[] getAllLists(String[] elements, int lengthOfList)
{

    //lists of length 1 are just the original elements
    if(lengthOfList == 1) return elements; 
    else {
        //initialize our returned list with the number of elements calculated above
        String[] allLists = new String[(int)Math.pow(elements.length, lengthOfList)];

        //the recursion--get all lists of length 3, length 2, all the way up to 1
        String[] allSublists = getAllLists(elements, lengthOfList - 1);

        //append the sublists to each element
        int arrayIndex = 0;

        for(int i = 0; i < elements.length; i++){
            for(int j = 0; j < allSublists.length; j++){
                //add the newly appended combination to the list
                allLists[arrayIndex] = elements[i] + allSublists[j];
                arrayIndex++;
            }
        }
        return allLists;
    }
}

public static void main(String[] args){
    String[] database = {"a","b","c"};
    for(int i=1; i<=database.length; i++){
        String[] result = getAllLists(database, i);
        for(int j=0; j<result.length; j++){
            System.out.println(result[j]);
        }
    }
}

Although further improvement in memory could be made, since this solution generates all solution to memory first (the array), before we can print it. But the idea is the same, which is to use recursive algorithm.

Java implementation of your permutation generator:-

public class Permutations {


    public static void permGen(char[] s,int i,int k,char[] buff) {
        if(i<k) {
            for(int j=0;j<s.length;j++) {

                buff[i] = s[j];
                permGen(s,i+1,k,buff);
            }
        }       
        else {

         System.out.println(String.valueOf(buff)); 

        }

    }

    public static void main(String[] args) {
        char[] database = {'a', 'b', 'c'};
        char[] buff = new char[database.length];
        int k = database.length;
        for(int i=1;i<=k;i++) {
            permGen(database,0,i,buff);
        }

}

}

This smells like counting in binary:

• 001
• 010
• 011
• 100
• 101
• ...

My first instinct would be to use a binary counter as a "bitmap" of characters to generate those the possible values. However, there are several wonderful answer to related questions here that suggest using recursion. See

• How do I make this combinations/permutations method recursive?
• Find out all combinations and permutations - Java
• java string permutations and combinations lookup
• http://www.programmerinterview.com/index.php/recursion/permutations-of-a-string/